Halftone screening is a printing technique that reproduces continuous tone images by using dots that vary in size or spacing. Halftone screening is used by many different printing technologies. The invention disclosed here is particularly valuable for screening applied to flexographic printing.
There are two common halftone screening methods used in the industry: Amplitude Modulation (AM) screening and Frequency Modulation (FM) screening. The most common screening method is AM, which positions dots on a fixed grid and varies the dot size according to the tone of the image. Various dot shapes can be used but round, elliptical, and Euclidian are the most common. FIG. 1A shows a typical AM screen with dots 104 positioned on the grid.
The principal disadvantage of AM screening for flexographic printing is the size of the smallest printing dot. The smallest relief dot that can be reliably created on a plate is about 20 to 30 microns in diameter. Any smaller and the dot may not form properly, leading to distortion and scum dot formation. In addition, the ink transferred from the plate to the substrate spreads out past the edge of the dot, increasing the dot diameter by another 20 to 30 microns. This results in a high minimum tonal value. For example, a 150 dpi screen will have minimum tonal value of about 10%.
A second method, often called FM screening, avoids a regular grid. Instead a stochastic process is used to place the dots. Initially the dots are of fixed size and the tone is increased by adding dots to the image. After a certain density is reached, the algorithm switches from dot addition to dot growth. The placement algorithm attempts to preferentially reduce the spatial frequency components in the screen to which the eye is most sensitive. FIG. 1B shows a typical FM screen with dot 108 positioned on the plate.
The main disadvantage of this method is the residual noise that is apparent in the mid-tones of the image. Mid-tones can be roughly defined as the tonal range from about 25% to 75% tone. This noise is often made worse when different color image separations are superimposed during the printing process. In this mid-tone range, AM screening usually out-performs FM screens.
The principal advantage of FM screens is their ability to achieve lower tonal values, when compared to AM screens, by continually removing dots from the screen. These lower tonal values in the range of zero to a few percent are referred to as the highlights. Similarly, the last few percent of tonal values up to 100% tone are referred to as the shadows. An ideal halftone screen would have the low noise characteristic of AM screens in the mid-tones combined with the extended tonal range of FM screens in the highlights. Several methods described in the prior art attempt to achieve this result, but with mixed results. There exists a need to create a hybrid screen without introducing any unwanted side effects.
In the prior art, the most straight forward method of extending the tonal range of an AM screen is to start removing dots after the minimum dot size has been reached. A stochastic algorithm is used to choose the order in which the dots are removed. The goal of the algorithm is to minimize noise in the low spatial frequencies of the screen. Some clustering of the remaining dots may be required to prevent scum dots from forming on the plate. FIG. 1C shows how this method is used in the highlight area of a vignette. Area 112 shows dots are removed from the AM grid. What is apparent from the figure is that the dot dropout causes voids 112 in the halftone screen that are easily visible. Many customers find such voids objectionable.
Better results can be achieved when a hybrid of AM and FM screening is used. A stochastic screen is used for the highlights areas while, in the mid-tones, an AM screen is deployed for noise reduction. There is an abrupt transition from FM to AM screen at the tone when all the AM cells are filled with a halftone dot of the minimum size. The algorithm ensures that a separation is maintained between AM and FM dots. This algorithm is an improvement over the simpler on-grid algorithm, however, it does sometimes suffer from a visible line appearing between the AM and FM regions. This can manifest as either a white line or a dark line between the regions. FIG. 1D shows an example of this type of screen. The voids 116 are not as apparent in the highlights
A further enhancement is to add a transition region between the FM screening in the highlights and the AM screening over the remainder of the tonal range. In this transition region, the screening method slowly changes from FM to AM and the average position of the halftone dots migrate toward the center of the halftone cell. FIG. 1E illustrates an example of such an implementation.